1. Field of the Invention
The invention relates to the field of devices incorporating hybrid microwave circuits.
2. Description of the Prior Art
The gain of hybrid microwave circuits is constantly increasing and to prevent feedback and/or oscillation phenomena the circuits must be installed in cavities that are isolated at microwave frequencies. The devices (or modules) in which they are installed therefore comprise a base provided with an external peripheral wall and at least one internal wall together delimiting at least two cavities able to accommodate hybrid microwave circuits and a cover adapted to be fastened to the edges of the walls to seal the cavities hermetically and to establish a grounding electrical contact with the walls.
To seal hermetically the whole of the hybrid circuit, the cover is welded to the external walls of the base, often by electrical welding using knurling tools or by laser welding.
As far as the internal wall is concerned, the tack-welding technique specific to closure by knurling tools provides a hermetic seal at microwave frequencies between the different cavities.
Because of the position of the internal walls, they are welded blind. Moreover, welding necessitates a wall thickness that is sufficient, firstly, to allow centering of the weld bead and, secondly, to prevent the projection of conductive balls of weld into the cavities and thus onto the hybrid circuits. This additional wall thickness is difficult to reconcile with the miniaturization and the weight reduction required in hybrid devices. Moreover, this additional wall thickness increases the length of the connections between the microwave subassemblies installed in the different cavities, which compromises the required electrical performance in microwave applications.
In an attempt to solve this problem, it has been proposed to fasten the cover to the internal wall or walls using an electrically conductive adhesive compatible with the microwave environment. However, this type of adhesive is mechanically weak, which is incompatible with the stresses that are applied to hybrid devices by the vacuum and by thermal cycling.
Placing an absorbent material in the cavities has also been proposed. However, this is only a partial solution to the problem and may compromise the reliability of some naked electronic dies of the hybrid circuits.
No prior art solution being entirely satisfactory, an object of the invention is therefore to propose an alternative solution intended to improve on this situation.